A panel of three federal judges have ordered the state to release more than 9,500 inmates by the end of the year to meet a court-ordered target for reducing prison crowding. Brown wants the U.S. Supreme Court to block the order while justices consider his appeal.

Brown, a Democrat, asked the former governors for their assistance, said Kent Scheidegger, the legal director at the Criminal Justice Legal Foundation and the author of the ex-governors' brief.

The prison system has been under court oversight because judges determined that overcrowding prevented adequate healthcare for inmates, causing death and unconstitutional conditions behind bars.

Brown and other officials have said the state has made enough improvements and warn that crime will increase if the state is forced to release inmates.

The former governors echoed those concerns.

"Given the irreparable harm of criminal victimization, the likelihood that further releases will cause more victimization, and the dramatic improvements in California prison healthcare since the original trial in this case, no further reductions should be ordered without a full examination of the case," the brief said.

The prison system is also facing other challenges, including a hunger strike by thousands of inmates and a separate court order to remove inmates from prisons where they face increased risk of valley fever.